01 -
Season the chicken breasts with salt and pepper on both sides. Heat olive oil in a skillet over medium-high heat. Cook the chicken for 6-7 minutes on each side until fully cooked and no longer pink in the center. Remove from skillet and let rest before slicing thinly.
02 -
Slice the avocado and prepare the cheese slices. Set aside.
03 -
Spread a thin layer of mayonnaise and Dijon mustard on each slice of bread.
04 -
Layer sliced chicken, avocado, cheese, red onion, and tomato on two slices of bread. Top each with the remaining slices of bread, mayo side down.
05 -
Heat a skillet or grill pan over medium heat. Add butter or olive oil to the pan. Cook sandwiches until golden brown and cheese is melted, about 3-4 minutes per side, pressing gently with a spatula.
06 -
Remove sandwiches from the pan and let cool for a minute before slicing in half. Serve warm.